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From: "codeeply at foxmail dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c++/65273] New: Incorrect output in SIMPLE program compiled from high version gcc with O2 Date: Mon, 02 Mar 2015 04:08:00 -0000 [thread overview] Message-ID: <bug-65273-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65273 Bug ID: 65273 Summary: Incorrect output in SIMPLE program compiled from high version gcc with O2 Product: gcc Version: 4.8.2 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: codeeply at foxmail dot com #include <iostream> #include <cstring> using namespace std; const int N = 2000017; int hash(const char *s) { volatile int ret = 1; int i, len = strlen(s); for (i = 0; i < len; ++i) { ret = ret * 131 + s[i]; } ret %= N; if (ret < 0) ret += N; return ret; } int hash1(const char *s) { int ret = 1; int i, len = strlen(s); for (i = 0; i < len; ++i) { ret = ret * 131 + s[i]; } ret %= N; if (ret < 0) ret += N; return ret; } int main() { cout << hash("sfihsifef") << endl; cout << hash1("sfihsifef") << endl; return 0; } --------------------------------------------------------------------- :~/test>g++ -O2 g.cpp :~/test>./a.out 602373 -1397644 --------------------------------------------------------------------- It's found that "if (ret < 0) ret += N;" was "optimized" without volatile. And this bug has been seen in 4.7.2 and 4.8.2, but not in 4.1.2.
next reply other threads:[~2015-03-02 4:08 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2015-03-02 4:08 codeeply at foxmail dot com [this message] 2015-03-02 4:25 ` [Bug c++/65273] " pinskia at gcc dot gnu.org
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